Commercially important isocyanates such as toluene diisocyanate (TDI) and methylene diphenylene diisocyanate (MDI) are usually prepared by reacting the corresponding diamines with phosgene. Because of toxicity concerns about phosgene, alternative non-phosgene routes to isocyanates are of interest. One non-phosgene approach to isocyanates is to crack dialkylureas in the presence of various promoters, as is taught in U.S. Pat. Nos. 4,871,871, 4,873,364, 4,883,908, and 4,978,779. Carbamates can also be cracked thermally or in the presence of promoters to give isocyanates. A disadvantage of these processes is that economical, selective routes to the required carbamates or dialkylureas from inexpensive starting materials are lacking, particularly for unsymmetric aromatic carbamates and ureas.
Oxidative dehydrogenation of N-methylformamide in the presence of a silver catalyst produces water and methyl isocyanate, as disclosed, for example, in U.S. Pat. Nos. 4,537,726 and 4,683,329. The process is effective for low-boiling isocyanates like methyl isocyanate, but is impractical for synthesis of other useful isocyanates like TDI and MDI because numerous side-products form. For example, U.S. Pat. No. 4,207,251 teaches oxidative dehydrogenation of formamides to produce TDI or HDI, but yields are 27-30%, and side products make the route commercially unattractive.
A conceptually more inviting route to isocyanates from formamides involves dehydrogenation of the amide without oxidation. The reaction products are an isocyanate and gaseous hydrogen rather than an isocyanate and water. Recovery of hydrogen in the isocyanate-forming reaction is a bonus because the hydrogen can be recycled for use in nitro-reduction step that begins the synthesis: ##STR1##
Watanabe et al. (J. Chem. Soc., Chem. Commun. (1990) 1497) reported a synthesis of N,N'-diphenylurea in 92% yield by dehydrogenating formanilide in the presence of aniline and a ruthenium catalyst: ##STR2## Watanabe et al. prepared numerous symmetric N,N'-diarylureas in this manner. In contrast, N-phenyl-N'-p-tolylurea was not obtained selectively from the reaction of formanilide and p-toluidine. Instead, a mixture of three ureas -- N,N'-diphenylurea (19%), N,N'-di-p-tolylurea (21%), and the unsymmetric product, N-phenyl-N'-p-tolylurea (38%) -- was obtained:
A selective process for producing unsymmetric carbamates or ureas is needed. Preferably, the ureas or carbamates are ones that can be easily cracked in the presence of a promoter to give isocyanates. Also preferred is a process that produces hydrogen, which can be used at an earlier step in the overall process.